The present invention relates to a centrifugal pump and an impeller thereof. The present invention especially relates to modifying an impeller of a centrifugal pump in such a way that the pump may be used without a risk of damaging a shaft seal or the like at capacities higher than that of the optimal operating point.
It is already known that when pumping liquid or a suspension by a centrifugal pump, liquid is entrained into a space behind the impeller of the centrifugal pump when working vanes of the impeller increase the pressure of the liquid in front of the impeller. Thereby, the liquid to be pumped in addition to being discharged through the pressure opening of the pump to the pressure conduit also tends to fill the space behind the impeller with a pressurized liquid. Although the liquid between the impeller and the rear wall of the pump rotates, on the average, half the speed of the impeller (provided that there are no so-called rear vanes or like ribs on the impeller shroud) and thus, while generating centrifugal force, reduces to a certain extent the pressure prevailing in the sealing space behind the impeller in the area of the shaft of the pump, a considerable pressure, however, as one would expect affects also the shaft seals in connection with the rear wall of the pump or behind it. Partially, therefore, so-called rear vanes have been arranged on the rear face of the impeller shroud, which rear vanes pump the liquid having entered the space outwards, whereby the pressure in the space behind the impeller substantially decreases.
The rear vanes must, however, be dimensioned so that they operate optimally only in a certain capacity range of the pump, whereby deviation in either direction from the capacity range results in that the pressure prevailing within the area of the rear vanes and also in the seal space changes. If the output of the pump is increased, the rear vanes generate, in the worst scenario, a negative pressure, which can, at its worst, also make the liquid in the seal space boil, especially when pumping liquids at a higher temperature. Correspondingly, when decreasing the capacity, for example, by constricting such by a valve, the pressure behind the impeller increases and the stresses increase. At the same time, as one would expect, the stress on the bearings also increases.
For a corresponding purpose, i.e. for balancing the pressure prevailing on the different sides of the impeller, it is also suggested that balancing holes be used, which are holes parallel to the axis of the pump made in the impeller shroud close to the hub of the impeller, through which the liquid from the side of the impeller where the pressure is higher is allowed to be discharged to the area of the lower pressure. In other words, the flow in the balancing holes may be in either direction.
However, although both balancing methods are in use, it has been noticed that when moving along a so-called pump curve in the H, Q (head, capacity) chart, i.e. to the right in the direction of higher capacity, the balancing in accordance with the prior art is not always capable of sufficiently preventing the pressure in the sealing space from dropping below the pressure prevailing in front of the impeller of the pump. This is problematic because the negative pressure in the sealing space leads to the fact that the lubricating effect of the liquid to be pumped or other liquid on seals decreases when the liquid escapes from the seals. Depending on the seal type, the escaping of the liquid from the seal may cause the seal to run dry, which with some seal types can very quickly lead to a damaged seal.
Another seal type to be used in the centrifugal pumps is a so-called dynamic seal, the operation of which is based on the operation of a rotor rotating in a separate chamber behind the rear wall of the pump. In favorable pressure conditions, the rotor comprising a substantially radial disc and vanes arranged on the rear surface thereof relative to the impeller of the pump rotates a liquid ring in the chamber in such a way that the liquid ring seals the space between the disc and the wall of the chamber, sealing at the same time the pump itself. If such a rotary liquid ring is subjected to a pressure difference high enough, the liquid ring will escape towards the lower pressure. If a pressure lower than that of the atmosphere is generated behind the impeller of the pump, it tends to draw the liquid ring out of the seal chamber. If this takes place, air is allowed to flow without problems from behind the pump into the pump. Air can also flow in a corresponding manner through the mechanical shaft sealing of the pump into the pump. The effect of the leaking of air on the pumping itself is that air, at its worst, stops the pumping.